1. Field of the Invention
The present invention relates to a wet friction material used for a clutch, a brake and the like in a friction engaging apparatus, and a manufacturing method therefor.
2. Description of the Related Art
FIG. 19 shows one example of the basic construction of a wet friction clutch. Torque is transmitted when drive plates 2 fitted into a spline 51 of a hub 5 fitted around an input shaft 6 comes into contact with driven plates 1 fitted into a spline 41 of a retainer 4. In this drawing, reference numeral 3 designates a pressure plate, and 7 is a pressure piston.
FIG. 16 is a perspective view showing the driven plates and the drive plates, and FIG. 17 is a side cross-sectional view showing the driven plates and the drive plates in combination. The driven plate 1 is made up of a steel plate portion 11 and a spline projection 12, whereas the drive plate 2 is made up of a steel plate portion 21, a spline projection 22, and a wet friction material 23 bonded to both sides of the steel plate portion 21.
FIG. 18 is a plan view of the drive plate 2, and a groove 24 which also acts as an oil reservoir is formed in the friction plate 23 bonded to the surface of the steel plate portion 21.
In view of the current energy and environmental issues, there is a demand for a compact and light-weight friction clutch which reduces operating shock and removes self-induced oscillation such as occurred shudder so as to provide good ride comfort as well as having a large torque capacity. The friction clutch is also required to cope with high energy resulting from increased rotational speed and output of an automobile engine. Thus, the demand is extremely great.
A conventional friction clutch employs many sophisticated controls in order to reduce fuel consumption and operating shock by increasing a continuous sliding state of a clutch during driving of a car, changing a duty efficiency, and controlling an engine so as to reduce the ratio of an input torque to a clutch capacity when a clutch is engaged.
The wet friction material comprises a fibrous base material such as natural pulp fiber and organic synthetic fiber, a filler such as diatomaceous earth, a friction adjustment agent such as cashew resin and a binder such as thermosetting resin. Conventionally, the binder contained in the inside of the friction material forms a high-concentration impregnated layer (a solid binder layer) on the front and rear layers (i.e., both sides) of the friction material.
The thermosetting resin, which is one example of binder, is commonly used as material which forms a wet friction material (composite fibrous paper). The resin of this type comprises phenol resin, epoxy resin, urea resin, melamine resin, silicon resin, or the like. A wet friction material produced by a paper-making method is widely known as the wet friction material. This friction material is manufactured by mixing a fibrous base material such as natural pulp fiber or organic synthetic fiber as fibrous base material with an agent for controlling friction, producing raw paper, and impregnating the raw paper with a diluted thermosetting resin solution, and evaporating the diluted solution in a drying process, and heating the paper to set the thermosetting resin.
The processes from the process of impregnating raw paper with binder to the drying process will be further explained. When the raw paper is impregnated with binder, the binder is diluted with an organic solvent to a predetermined concentration. After the raw paper is sufficiently impregnated with the diluted binder, the organic solvent is evaporated in a drying process. However, the binder is captured by surface tension, whereby an excess binder coating and a high-concentration binder layer are formed along the outermost layer (about 100 xcexcm) of the friction material surface. It has been impossible to prevent the high-concentration binder layer from being formed along the outermost layers of the front surface (a front layer) and the rear surface (a rear layer). The rear surface and the rear layer are the side of the friction material which is not bonded to the friction surface but bonded to the steel plate. The binder is thermally set in the thermosetting process, as a result of which the binder coating and the high-concentration binder layer formed along the friction material surface are fixed.
It is found that the influence of the excess binder coating and the high-concentration binder layer formed along the outermost layer of the surface by the physical properties (surface tension) of the binder brings about the following problems:
(1) When in an initial state, the binder coating formed over the fibrous base material of the outermost layer is hard and less flexible and forms minute projections. Hence, the binder coating is not necessarily smooth, and only the projections of the binder coating form a sliding surface in view of a macroscopic point when the friction material is in contact with the driven plate (a corresponding sliding surface). For this reason, since a small contact area between the driven plate and the binder, and small original coefficients of friction of the binder, the driven plate result in a small coefficient of friction in the initial state. The binder wears out through the repetition of sliding action, as a result of which a soft fibrous base material is uncovered. This increases the contact area and makes the sliding surface smooth, thereby resulting in an initial running-in state in which the thus uncovered fibrous base material having a large coefficient of friction increases the coefficient of friction between the binder and the driven plate.
(2) The surface of the friction material has a high concentration of binder and lacks flexibility and smoothness, and hence contact between the friction material and the driven plate becomes uneven, thereby bringing about a microscopic wedge effect of an oil film. This wedge effect causes increased operation shock and shudder occurs.
(3) Since the surface of the friction material has a high concentration of binder, the friction material is prone to turn into plastic as a result of a sharp increase in temperature.
Through this running-in process, a torque capacity of the friction material changes from its value which is originally set when the friction value was new during a very short period of time. For this reason, the running in process is considered as a significant quality problem.
FIG. 8 is an explanatory view schematically showing the construction of a surface area of a conventional friction material, and FIG. 9 is a surface contour line R showing the enlarged surface of the friction material. In these drawings, A is a binder (resin) part, B is a fiber part, and C is a filler. As can be seen from the drawings, the surface contour line R is not smooth (this conventional example is designated by L2).
As one example, FIG. 5 shows the distribution of binder L2 in a thicknesswise direction inside of a common friction material after it has set. In view of a product, the problem is that if the capacity of a clutch is designed based on a small friction coefficient of new friction material, a torque capacity increases as the friction coefficient varies in time sequence as a result of a running-in process, thereby bringing about operating shock. For an expensive luxury car, an extra learning function might be added for control. With a low friction coefficient of new friction material, the surface temperature of the friction material increases by frictional heating resulting from extension of a slid time under harsh driving environments. This, in turn, promotes the turning of the binder (thermosetting resin) into plastic because many binders are distributed around the surface layer a phenomenon in which the thermosetting resin around the surface of the friction material resets or becomes carbonized by frictional heating, so that a frictional surface becomes a mirror surface). The changing of the binder to plastic brings about a fading phenomenon, which in turn causes a further extreme drop in friction coefficient. In this way, the problem of heat resistance and durability is encountered. An additive contained in lubricant oil is decomposed and separated out by the friction heat. The thus separated additive attaches to the surface of the friction material and a corresponding slide surface, which clogs the surface of the friction material. As a result of this, the friction material fails to offer its original performance, thereby resulting in a similar drop in friction coefficient.
To prevent these problems, an actuating pressure might be increased to reduce a slide time. However, this method also brings about the following problems: namely, a drop in peel life of the friction material due to repetitive compressive-fatigue caused by a high surface pressure, the generation of a heat spot in a corresponding friction surface (the driven plate) resulting from an increase in heat rate per unit time, thermal deformation, an increase in the size of a hydraulic pump for generating a large hydraulic pressure, and a problem of durability and lifetime such as actuating fluid leaks.
Where the amount (concentration) of impregnated binder is increased to improve the peel life and strength of the friction material, several problems arise as follows: a deterioration in frictional properties (operating shock and occurred shudder) caused by the lack of flexibility of the friction material; a deterioration in a running-in process in which the friction coefficient varies by the influence of the binder layer along the surface of the friction material after the friction material has undergone engaging action several times since it was new; and adhesion of an additive to the friction material as a result of shaving of the additive of lubricant oil attaching to a corresponding slide surface by fibers of the surface of the friction material which are fixed by the binder. As previously mentioned, any of these problems are ascribed to the binder coating and the high-concentration binder layer formed along the outermost layer of the friction material when the friction material was new.
To solve these problems, an excessive binder coating formed along the surface layer of the friction material is conventionally removed by sliding the friction material for a predetermined time, or by machining the surface of a new friction material (as disclosed in Unexamined Japanese Patent Publication (kokai) No. Hei-5-99297). However, these attempts resulted in a considerable drop in durability an lifetime due to a drop in strength by cutting fibers as shown in FIG. 10. Some other methods are also employed wherein the surface of the friction material is carbonized by smoothing a heat plate (as disclosed in Unexamined Utility Model Publication (jikkai) No. Sho-62-149629) or the surface is forcibly smoothed. However, the former method induces a drop in life of the friction material because of a drop in strength of the friction material as a result of the carbonization of fibers. On the other hand, the latter method results in a smooth surface of the friction material, but the smoothing of the surface does not lead to the removal of the excessive binder coating. Thus, this method also fails, to provide a fundamental counter-measure against the fading phenomenon and occurred shudder, which in turn adds to product costs.
It is an object of the present invention to provide a wet friction material being capable of reducing its coefficient of friction without lowering its strength and reducing operation shock.
A wet friction material of the present invention is comprised of a fibrous base material, a filler, a friction adjustment agent and a binder; wherein a ratio A/B is in the range of 0.85 to 1.15 where A is a binder concentration from a surface of the wet friction material to a depth of 10 xcexcm and B is a binder concentration from a depth of 10 xcexcm to a depth of 100 xcexcm.
Further, a method for manufacturing a wet friction material of the present invention is comprised the steps of: impregnating a raw paper comprising a fibrous material, a filler and a friction adjustment material with a binder; removing an excessive binder coating and a layer containing much binder, which are formed in an outermost surface layer during the impregnating step, by sucking means; drying the binder in the raw paper; setting the binder in the raw paper; and smoothing at least one surface of the raw paper.
According to the present invention, the excessive resin coating and the resin layer formed along the outermost surface (having a thickness of about 100 xcexcm) of the friction material of the present invention are reduced. Therefore, the total amount of resin is not affected by this reduction so as to dramatically improve thereby the conformability of the friction material. Further, since the excessive resin coating is few in the surface layer, the fading phenomenon and the changing of the friction material to plastic are prevented. Still further, the surface layer of the friction material has high flexibility, and therefore the friction material can reduce operating shock. The ratio of friction between the resin and a corresponding slid surface is reduced, and hence the friction material possesses a high coefficient of friction. For this reason, it is possible to alleviate the influence of the transfer of additives included in the lubricant oil upon the friction material.